Erection(tumescence) is a complex neurophysiologic event that leads to relaxation of the smooth muscle cells in the corpus cavernosum penis which allows filling of blood into the cavernous spaces causing an increase in the size and rigidity of the penis. Flaccidity (detumescence) is initiated and maintained by contraction of the corpus cavernosum smooth muscle (CCSM) cells. The CCSM is unique since it remains in the contracted state most of the time. It is not know whether the Ca2+-calmodulin-MLCK pathway or other then filament- mediated regulation (via the actin-binding protein caldesmon) plays a role in keeping these cells in the contracted state, for the penis to remain flaccid. A third possibility is that the myosin isoforms in CCSM play a role in determining the velocity of force generation and the tonicity. A recent study conducted by this investigator showed that the aortic smooth muscle, which is considered tonic, contains one type of myosin isoform whereas the smooth muscle cells in small muscular arteries contain a different myosin isoform. The latter isoform is encoded by a myosin heavy chain mRNA that is alternatively spliced to insert a 21-nucleotide sequence that encodes a region near the ATP-binding site on the myosin head. This insert encodes a 7-amino acid peptide, and the resulting myosin isoform has a two-fold increase in the actin-activated myosin ATPase activity and an associated increase in shortening velocity (Vmax). Similar studies conducted on CCSM in our lab show that the CCSM contains both inserted (as in small muscular arteries and all visceral smooth muscles which show phasic characteristics) and non-inserted (as in aortic smooth muscle which is tonic) myosin heavy chain isoforms. Specially, using CCSM from normal rabbits, normal humans, and patients with erectile dysfunctions, we will address the following questions: (1) Are the differences in the contractile characteristics of CCSM compared to smooth muscle from other sources due to a difference in the composition of smooth muscle myosin isoforms that are produced by alternative splicing of the myosin pre-mRNA at the 3' end (resulting in SM1 and SM2) or the 5' end (resulting in inserted myosin)? (2) What role does myosin light chain phosphorylation play in erectile function and do CCSM cells have higher kinase or lower phosphatase activity compared to other smooth muscle cells (e.g. bladder smooth muscle)? (3) What is the level of expression of myosin light chain kinase and caldesmon in the CCSM and is it altered in patients with erectile dysfunction? Thus, the results from experiments proposed in this application will provide the molecular mechanisms for the regulation of CCSM contraction and delineate specific steps in the pathways for cross-bridge cycling and contractility of smooth muscle cells in the normal corporus cavernosum penis. It will also provide and explanation for changes in contractility of smooth muscle cells seen in patients with erectile dysfunction.